1. Field of the Invention
The invention pertains to polymeric torque transfer devices such as chain sprockets or gears for transferring torque from one shaft to another wherein the body and the teeth of the device defined on the periphery thereof are formed of a polymeric material.
2. Description of the Related Art
Chain sprockets for association with roller and segmented chains, or gears with meshing gear teeth, are commonly employed to transfer torque between rotating shafts. By varying the diameters of the sprockets or gears, the ratios of rotation between such interconnected torque transfer devices can be regulated. Typically, the sprockets or gears are formed of metal and may be stamped, machined, or molded of powdered metal, and as the chain or gear teeth are also formed of metal, considerable noise and wear exists between the rotating sprocket or gear, and the chain or teeth.
In order to reduce costs, and reduce noise and wear, chain sprockets, and gears, have been formed of polymeric materials such as nylon or the like, and upon using a polymeric material having self-lubricating characteristics, costs, weight, noise and wear can be reduced. However, because of the relatively "soft" characteristic of polymeric materials, polymeric sprockets and gears formed solely of a polymeric composition can only be used in light duty applications as the material is not capable of transmitting higher torques without experiencing excessive stress and damage. Higher torques cannot be transferred by conventional polymeric devices as occurs in automotive timing chains such as used with engine camshafts, chain drive gear systems, motorcycle and bicycle chain drives, agricultural and industrial drives and gear transmissions without some means to remove concentrated stresses at the hub.
In order to accept the hub stresses in polymeric sprockets and gears, such devices are usually molded over a metal insert hub. Such inserts are used in the hub of the polymeric device to increase the strength and torque transfer capabilities of the device decreasing the probability of long-term deformation and loss of function. However, when an insert is molded into a polymeric sprocket or gear, the polymeric material shrinks during cooling producing high gripping forces adjacent the insert. While such shrinkage of the polymeric material tends to grip the insert strongly, it leaves considerable residual stress in the polymeric material and this stress, over time, often results in failure of the polymeric material due to the presence of such stress points. Further, as polymeric material shrinks at a higher rate of displacement than the metal insert, this difference of coefficient of expansion of the polymeric material and the metal insert changes the mechanical bonding relationship between the metal and non-metal components often resulting in cracking of the polymeric material around the metal insert producing failure of the device.
Accordingly, because of the differences of coefficient of thermal expansion, and the producing of stress points adjacent the insert, torque transfer devices formed of polymeric material utilizing metal inserts have not been suitable in the effective transfer of torque through polymeric sprockets and gears wherein the torque requirements are relatively high, especially in applications where temperature extremes occur.